Field of the Invention
The present invention relates to a vapor drying apparatus for a substrate, a vapor drying method for a substrate, and a vapor supplying apparatus and a vapor supplying method used for vapor drying process on a substrate. The substrate mentioned herein includes various types of substrates such as a semiconductor wafer, a glass substrate for a photomask, a glass substrate for a liquid crystal display, a glass substrate for a plasma display, a substrate for an field emission display (FED), a substrate for an optical disk, a substrate for a magnetic disk, and a substrate for a photoelectromagnetic disk.
Description of the Background Art
In a step of manufacturing an electronic component such as a semiconductor device or a liquid crystal display device, a substrate such as a semiconductor wafer or a glass substrate for a liquid crystal display device is subjected to process using processing liquid. More specifically, a main surface of the substrate is cleaned or etched by being subjected to wet process with a chemical liquid. Then, pure water such as de-ionized water (hereinafter called “DIW”) is supplied to the main surface of the substrate having received the chemical liquid to perform rinsing process of washing off the chemical liquid from the main surface of the substrate.
After the rinsing process, drying process is performed to remove the pure water remaining on the main surface of the substrate to dry the substrate. For example, a conventionally employed method for this drying process includes a spin-drying method of shaking off and removing the pure water using centrifugal force resulting from rotation of the substrate and a blow-drying method of blowing off or evaporating the pure water on the main surface of the substrate and removing the pure water by blowing nitrogen gas on the main surface of the substrate.
However, the aforementioned drying methods cause the risk of pattern collapse if a pattern made of fine projections and depressions is formed on the main surface of the substrate. The pattern collapse is caused by the surface tension of the pure water left unremoved in a place inside the pattern such as a depression of the pattern as the pattern on the main surface of the substrate is exposed from the surface of the pure water in response to progress of the drying. Prevention of the pattern collapse has become an important problem to be solved particularly in recent years where a pattern on the main surface of the substrate has become finer and finer.
According to one technique, liquid or vapor of isopropyl alcohol (hereinafter called “IPA”), which is an organic solvent of lower surface tension than pure water, is supplied to a main surface of a substrate after being subjected to ringing process to replace pure water adhering to the main surface of the substrate with the IPA. Then, the substrate is dried by removing the IPA from the main surface of the substrate.
A technique of drying a substrate by supplying vapor of IPA to a main surface of a substrate to replace pure water adhering to a main surface of a substrate with the IPA and then removing the IPA from the main surface of the substrate is described in Japanese Patent Application Laid-Open Nos. 4-155924 (1992) and 2008-198741.
The following describes known methods of generating vapor of IPA. The method of Japanese Patent Application Laid-Open No. 2003-168668 employs a bubbling system of generating a mixed gas containing IPA vapor by bubbling an N2 gas through IPA stored in an IPA tank. The method of Japanese Patent Application Laid-Open No. 2007-46838 employs a two-fluid nozzle system of generating IPA vapor by generating a mixed fluid of an IPA liquid and an N2 gas using a two-fluid nozzle and then by heating the mixed fluid. The method of Japanese Patent Application Laid-Open No. 5-90240 (1993) employs a liquid heating system of evaporating IPA in a liquid state stored in a tank by heating the IPA with a heater block.
Japanese Patent Application Laid-Open No. 2013-23440 discloses a technique of recycling waste IPA having been used in a step of manufacturing a semiconductor device by collecting the waste IPA and purifying the collected IPA to a level comparable to the level of the IPA having been purchased in preparation for the step of manufacturing the semiconductor device. According to the technique disclosed in Japanese Patent Application Laid-Open No. 2013-23440, the collected IPA is subjected to ion-exchange process of removing an ion component in the IPA using an ion-exchange resin, dewatering process using a pervaporation membrane, and distilling process performed sequentially. Then, the aforementioned purifying process is performed on the waste IPA in a circulation line until an alcohol concentration in the IPA becomes a certain level or more. The purified IPA is thereafter supplied to a supply tank.
As described above, in a step of manufacturing a semiconductor device, prevention of pattern collapse resulting from the surface tension of water left unremoved inside a pattern on a main surface of a substrate is a problem to be solved while the substrate is dried. This problem has conventionally been solved by replacing the water inside the pattern with a substance of low surface tension such as IPA. However, under the influence of water contained in IPA itself to be supplied to the main surface of the substrate, water still remains on the main surface of the substrate even after the replacement with the IPA. This makes pattern collapse uncontrollable in recent years where a pattern has become finer and finer.
More specifically, if water remains on the main surface of the substrate, the water is filled in a gap between pattern elements formed on the main surface of the substrate. If the filled water reaches a height sufficient to exert surface tension relative to the height of a projection of the pattern, the surface tension of the water in the gap between the pattern elements acts on the pattern as this water is vaporized, causing collapse of the pattern. The volume of the gap between the pattern elements is reduced with increase in the fineness of the pattern. Thus, even if the water in a tiny amount remains on the main surface of the substrate, the water is still filled in the gap between the pattern elements to a height sufficient to exert surface tension.
Thus, for prevention of pattern collapse, the amount of water remaining on the main surface of the substrate should be reduced further in response to increase in the fineness of the pattern.
Referring to Japanese Patent Application Laid-Open No. 2013-23440, for example, a circulation line for dewatering is provided in a line of collecting IPA after being used for substrate processing. Thus, in Japanese Patent Application Laid-Open No. 2013-23440, water originally contained in IPA charged as refill alcohol in a supply tank cannot be removed from this IPA, thereby causing the risk of pattern collapse due to this water.
According to a technique disclosed in Japanese Patent Application Laid-Open No. 2008-112971, for example, while processing space in a chamber where a wafer is to be processed is filled with clean dried air (CDA), an IPA liquid is supplied to a wafer to replace pure water on the upper surface of the wafer with the IPA, thereby suppressing mixture of water into the IPA liquid supplied to the surface of the wafer.
In some cases, however, water in large amount is contained in IPA in a stage before being supplied to a substrate, specifically, in the IPA in a liquid state stored in a tank waiting to be supplied from a nozzle to the substrate. In these cases, the large amount of water is already contained in generated vapor of the IPA. Thus, drying the substrate by blowing this vapor on the substrate causes pattern collapse due to the water.
As a result of the high hygroscopicity of IPA, while IPA is brought in and poured into a tank or while the IPA is stored in the tank, the IPA contacts external air containing moisture and absorbs the moisture. In this case, a water concentration in the IPA might be increased to a predetermined concentration value or more before the IPA is used for substrate processing.
Even if the aforementioned absorption of moisture is substantially unlikely during incoming transport of IPA, for example, the risk of pattern collapse is still caused due to water originally contained in the IPA at the time of its purchase in recent years where a pattern has become finer and finer.
Commercially-available IPA is classified into grades in a manner that depends on an intended purpose of use, for example. Values relating respectively to impurities such as water content are specified for each grade. For use in electronics industrial purposes such as a step of manufacturing a semiconductor device requiring IPA of higher purity, IPA of what is called “EL grade (high grade)” is used. The concentration of IPA at this grade is about 99.9% or more, although this concentration differs depending on respective specified values of manufactures. However, even IPA at the EL grade may still contain from about 0.01 to about 0.1% of water. Even water in such a tiny amount still causes a risk leading to pattern collapse.